Mobile devices, which will hereafter be called robots, need some form of energy source for propulsion, movement, and other functions. This has been accomplished by several means in the past. The most common way is to have a storage system, such as a battery, which stores electrical energy. Another common method is to have wires attached to the robot providing the energy supply.
Along with the problem of providing power to the robot, there is the need to transmit control signals. It may also be desirable to receive data signals from the robot. Radio broadcasting has been used for signal transmission, as have infrared links. Each of these methods has some drawback. Batteries as a power source are usually heavy, adding considerably to the overall size of the robot. Tethered robots are limited in their range, broadcasting entails extensive specialized circuitry, while infrared links require a direct line of sight transmission path.